In the case of an amplifier having a triode/ultralinear switch, the ultralinear position will, roughly speaking, usually double maximum power capability compared to the triode position (i.e., a 3 db increase), while probably also affecting sonics in some manner that depends on the design of the specific amp. If an amp you may consider provides adequate power for your purposes but lacks such a switch I wouldn’t consider the lack of the switch to be significant.
Also, FWIW, the only amps I’ve had which provided such a switch were two classic oldies, Marantz models Two monoblocks (late 1950s) and Nine monoblocks (1960s), which I owned during the 1990s. In both cases I preferred the sonics of triode mode, although the 18 or 20 watt capability of the Two in triode mode was not sufficient to handle some classical symphonic recordings having particularly wide dynamic range, with the 90 db speakers I had at the time.
Re fixing locally vs. shipping to the manufacturer, I suppose factors to consider would be the weight of the amp and (if it is bought used) whether or not original packing materials are available, either from the seller or by ordering them from the manufacturer. But as a general rule of thumb I would say it is preferable to have an amp fixed by the manufacturer.
Best regards,
-- Al
